Electrographic paper for use in electrostatic printing, said paper being formulated to look like ordinary paper, consists of a paper substrate generally made conductive by the addition thereto of electroconductive resins or salts and on the top of which is placed a highly resistive dielectric resin coating. Generally these papers contain a pigment, the purpose of which is to reduce the gloss of the polymer coating and to give the paper a flat, matte-type finish. The resulting paper can be of any size, depending upon the requirements of the machine.
In a typical printing process, the paper is passed through an electrographic printer. Voltage in the range of 100-900 volts is applied across the dielectric coating. One process has an electrode making contact with the electroconductive substrate, and the other electrode, consisting of styli which are selectively activated by programmed impulses, is in proximity with the dielectric coating. An electrical discharge occurs across the air gap, resulting in an electrostatic image on the surface of the paper.
An alternative process charges the dielectric layer through one stylus, with the return path through another electrode on the same side of the paper.
The dielectric layer functions as a charged capacitor, the charged paper then being passed through a toner containing oppositely charged particles, The particles adhere to the electrostatic image, resulting in a visible print.
There are many dielectric materials which, when coated on paper at about 50 percent room humidity, will accept a static charge and produce a toned image. Any fairly good dielectric will hold sufficient charge long enough to be toned out in a few minutes to produce a visible print.
But when the coated papers are run through high speed printers (e.g., 5,800 lines per minute) and when the humidity to which the paper is subjected begins to vary over a range of about 10 to 85 percent, each component of the electrographic system begins to have critical requirements.
At low humidity the conductivity of the base sheet must be provided solely by the electroconductive resin with which it is impregnated. Conductivity of the base sheet determines the time required to transfer charge to the dielectric. The dielectric coating must be capable of being charged in a matter of about 50 microseconds. As the humidity goes up to 80 percent and higher, other problems become paramount. For instance, the papers begin to exhibit curl problems and the charge leaks off and through the paper much more quickly. In addition, parts of the dielectric coating lose their dielectric strength and break down at voltage levels which are inadequate for printing.
Past experience has shown that many soft resins, such as vinylidene chloride copolymers, while they do not curl the paper excessively, will not adequately receive and maintain an electrical charge under high humidity conditions. On the other hand, while many hard resins, such as epoxy resins, will receive and maintain a charge, they exhibit excessive curl under these conditions.
Coatings prepared from blends of vinylidene chloride polymers and hard resin such as epoxy resins, have been found to provide desirable charge acceptance and retention along with good image density, resolution and appearance, under conditions of relatively high humidity. Such coatings, however, still tend to curl and the polymeric ingredients of such coatings have only limited solubility in conventional organic solvents from which such coatings are cast with resultant requirements for recovery of relatively large amounts of such solvents.